As is known, cast iron melts are treated with pure magnesium or magnesium intermediate alloys in order to achieve a spheroidal or vermicular formation of the graphite in the cast iron matrix and thereby to influence in an advantageous manner the mechanical-technological properties of the properties of the workpieces cast therefrom.
In the case of the production of cast iron with spheroidal graphite or vermicular graphite, the after-treatment of the cast iron melts by inoculation with special inoculation alloys is conventionally a part of the production technique in order to satisfy the increasing demands of quality, in which case especially ferrosilicon alloys (DIN 17560; company brochure of GfE Gesellschaft fur Elektrometallurgie mbH, Dusseldorf, May, 1989; company brochure of Metallgesellschaft AG, Frankfurt, Metallurgie and Giessereitechnik, June, 1979, pp. 10-11) are very frequently used inoculation agents. The danger of carbide formation is prevented by the nucleation action of the inoculation agent. The action is based on the fact that the inoculation agent forms nuclei for the separation of the graphite. Furthermore, due to the high silicon content in the inoculation agent, the solubility of the carbon is locally reduced so that the separating out of the graphite in the case solidification is made easier. An overcooling of the cast iron melts is clearly reduced, the number of eutectic cells or spherulites is increased and thus the microstructure is more finely grained. The small addition of inoculation agent of about 0.05 to a maximum of 1.0% corresponds to a take up of silicon by the melt of 0.05 to 0.80%. For the limitation of the silicon take-up, as well as of the temperature loss of the melts, the aim is, inter alia, to use small amounts of inoculation agents but very effective inoculation agents. By means of the addition of inoculation agents, mechanical and physical properties, such as tensile strength, toughness and elasticity, are improved.
Since the inoculation effectiveness of the inoculation agent is subject to a chronological diminution effect, the addition of the inoculation agent should take place as shortly as possible before the solidification, for example by the use of the mould treatment process. It is also known to place powdered ferrosilicon alloys into comparatively thin-walled hollow wires of steel, copper, nickel or aluminum alloys (company brochure of INFORM-Impfdraht, Chemetall GmbH, Frankfurt, March, 1988). The inoculation wire is rolled off at a constant speed into the cast iron melt or is introduced into the pouring stream of molten metal in the case of the pouring off of the melt. Since the end of the inoculation wire to be melted off is present in the cast iron melt or pouring stream of molten metal, an ideally uniform addition and a controlled distribution of the inoculation agent in the melt takes place.
A diminution effect also occurs in the case of the treatment of the cast iron melt with magnesium or magnesium alloy which is the stronger when, under operational conditions, the time interval between the treatment and the casting of the melt is greater than after the treatment of the melt with an inoculation agent. For this reason, the treatment of the cast iron melt with magnesium or magnesium alloy is always to be carried out with an excess of magnesium. This excess is only of limited effectiveness since the diminution effect is simultaneously increased.